- 1. 2. 3. #pragma once. ... warning: #pragma once in main file.
- Using #pragma once will delegate the task, of detecting subsequent #include statements for the same file, to the compiler. It can do this efficiently and safely.
- The #pragma once directive, once present in a file, assures that the file will not be included multiple times in the current project.
- On the other hand, since with #pragma once files are excluded based on their filesystem-level identity, this can't protect against including a header twice if it exists...
- So, instinctively, we can think that the #pragma once directive does the job of a header guard, but with only one line and without having to think of a macro name.
- #Pragma once is compilation-related, that is, it can be used on this compilation system, but not necessarily in other compilation systems, which means that...
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- (3) # pragma once (commonly used). You only need to add this command at the beginning of the header file to ensure that the header file is compiled once.
- There are two idiomatic methods to prevent a file from being included multiple times within the same translation unit: #pragma once and a guard macro using #ifndef.
- The problem with this is that it does not work, since pragma once directive is only effective for the file it is located in, in this case pragma_once.h.